Elastic synthetic-resin compounds for use as adhesive and sealing compounds for absorbent and non-absorbent undercoats or surfaces have gained increasing importance in recent years, especially in the building sector. While Thiokol sealants have predominated in the past, silicone and polyurethane sealing compounds have been coming into wider use of late.
Elastic sealing compounds are used especially to seal the joints of prefabricated structural elements and exposed concrete surfaces and, in addition to their sealing function, must compensate for considerable temperature-dependent movements. This expansion and contraction can impose severe stresses on the adhesion surface. Such sealants are also being used as caulking compounds on windows, between the window frame and the glass pane and between the masonry and the window frame, as well as in the installation of sanitary fixtures.
In these applications, too, the adhesive capacity of the sealing compounds has to meet stringent requirements.
Good adhesion is usually achieved by priming the substrates to be bonded or the surfaces to be sealed with a special coating which bonds to both the substrate and the sealant and thus is able to exert adhesion-promoting action.
The purpose of such primers is to provide the adhesion between substrate and sealant which otherwise would be lacking or inadequate.
While such primers are absolutely essential to successful application of sealing compounds, many users feel that using them is expensive and troublesome.
Improved adhesion of sealing compounds and greater handling ease are possible when special additives known as adhesion promoters are used. These generally are special organosilicon compounds which contain, in addition to a group that is reactive with respect to the polymer, two or three alkoxy groups which are attached to the silicon atom, for example: EQU (R'O).sub.3 --Si--(CH.sub.2 --CH.sub.2 --CH.sub.2).sub.a --Z
wherein
R'O is an alkoxy (e.g., methoxy or ethoxy) group,
Z is a functional organic group, and
a is one.
It is generally believed that the adhesion-promoting effect is due to a reaction of the additives with both the polymer and the substrate. The reaction with the substrate, for example, glass, occurs through hydrolysis of the Si--OR' bonds and condensation of the Si--OH group of the adhesion promoter with a Si--OH group of the glass surface, for example. In addition, however, there are also physical interactions between the adhesion promoter and the surface. The reaction with the polymer occurs through the reaction of the functional organic group Z with the reactive group of the polymer. (See DEFAZET, Deutsche Farben-Zeitschrift, reprint from No. 5, pp. 207-211.)
These adhesion-promoting additives are preferable to the use of adhesion-promoting coatings since they substantially reduce the work that has to be done on the construction site.
Frequently, however, the effect of these adhesion-promoting additives still falls short of meeting practical requirements, especially in the case of silicone an polyurethane sealing compounds. With polyurethane sealants, for example, it has not been possible heretofore to dispense with the use of adhesion-promoting coatings because the usual additives have not proved fully satisfactory.